This invention relates to the recovery of petroleum from subterranean petroleum reservoirs and more particularly to an improved miscible flooding process for recovering petroleum from subterranean formations.
It has long been recognized that substantial amounts of petroleum remain unrecovered in the reservoir at the completion of normal primary recovery operations. Hence, various secondary methods of recovering additional quantities of oil have been proposed, such as the well-known technique of waterflooding in which water is injected into the reservoir through an injection well to drive additional oil toward one or more production wells spaced apart in the reservoir from the injection well. Although an additional quantity of oil often can be recovered by waterflooding, the efficiency of the waterflood and the ultimate oil recovery can be further improved by introducing into the reservoir prior to the floodwater a petroleum displacement system that is miscible with both the reservoir oil and with the floodwater.
These displacement systems are generally admixtures of a liquid hydrocarbon, one or more surface active agents, and a stabilizing agent and may contain water up to the concentration at which the water-in-oil microemulsion inverts to an oil-in-water emulsion. Alternatively, the displacement system may comprise a separate slug of the liquid hydrocarbon and admixed agents followed by a slug of low salinity water which may also include a thickening agent and the like.
While flooding processes utilizing a miscible displacement system (often referred to as "micellar flooding,", "soluble oil flooding" or "tertiary recovery") can obtain high oil recoveries under controlled laboratory conditions, it has been found that under field conditions petroleum recoveries are substantially lower than would be expected from the laboratory tests and, in some cases, may be so low as to render the recovery operation uneconomical. One major factor in the reduced petroleum recovery efficiency under field conditions may be the incompatibility of the miscible system with certain cations encountered in the formation, such as calcium, magnesium, barium, iron and the like. Recognizing that the presence of these cations in the reservoir may be harmful to the displacement system, certain procedures such as preflushing have been suggested as effective in reducing the cation content in the reservoir by diluting or displacing the reservoir free water prior to the injection of the solvent system. However, such treatments do not completely eliminate the multivalent cation content of the reservoir clay or of the connate water which is bound as a film about the grains of the granular reservoir rock. Consequently, enough cations are still present to adversely affect the micellar slug.